1. Field of the Invention
This invention relates to RF power amplifiers, and in particular, to such amplifiers operated in a mode characterized by high quiescent-state current and highly peaked voltage at an active device output.
2. Related Art
Since the early days of vacuum tubes, RF amplifiers have been classified using the letters A, B and C, either singly or in combination. These amplifier classes have remained in common usage, in essentially unmodified form, despite a continually changing technology for electronic amplifying devices. Semiconductor devices, such as bipolar transistors, and later field effect transistors (FETs), although physically very different in their modes of operation from tubes, can be classified in much the same manner as that originally conceived for vacuum tube amplifiers.
Over the last few years, there has been a rapid growth in a type of consumer electronic products that use RF amplifiers and are generally referred to as personal communications service products. The cellular telephone is an example of such a product. Originally, cellular telephone service was intended for use in automobiles. More recently, the product is made as a small, even pocket-sized unit that a person can conveniently carry with them.
In order to reduce the size and weight of the units, the weight and size of the battery must also be reduced, while maintaining the time between battery replacement or recharge. One avenue is to make smaller, higher capacity batteries. Presently, the most effective batteries for these products have voltage outputs in the range of 1 to 3 volts. Circuitry designed to work with these small voltages also tend to be smaller and lighter.
The size of the units can also be made smaller and lighter by making the internal circuits more efficient and capable of operating at the reduced voltages. In a typical product, most of the power is consumed by the transmitter output amplifier, which typically boosts the telephone voice signal from about a 1 milliwatt level to a maximum power level, usually one-half to one watt. Any improvement in efficiency of the output amplifier results directly in a corresponding extension of the battery lifetime, thereby improving the market potential of the product.
A basic RF amplifier uses an active device, such as a tube, BJT or FET. The device is responsive to a low-level input control signal applied to a control terminal to cause a flow of current from a DC supply, such as a battery, through the active device. If a load resistance of a suitable value is placed in series with the DC supply, an amplified replica of the input signal appears across the load resistor. The active device typically has a range of input control voltage over which the supply current can be controlled in an approximately linear fashion. This range is typically from near zero to a maximum value, I.sub.max. For maximum linear power output, then, the drain current swings between zero and I.sub.max, and the device voltage swings between zero and V.sub.dc, the supply voltage.
The efficiency of such basic, linear amplifiers is very low. The class A amplifier improves on the efficiency of the basic amplifier by providing a separate, low resistance path for the DC component of current. This is typically achieved using a high reactance choke which presents a very high impedance to the RF signals, but allows DC to pass with negligible resistance.
Assuming the active device is an FET, a DC bias equal to half the cutoff point, referred to as the pinchoff voltage, is applied to the control terminal or gate. The current through the active device again varies between zero and I.sub.max, but the RF voltage varies from zero to 2 V.sub.dc. Such a device then has a drain efficiency (RF power/DC power) of 50%. Thus, although it is a linear amplifier, half of the DC power supplied is dissipated as heat in the active device.
A class B amplifier increases the efficiency further. With this class of amplifier, the gate is biased near the cutoff point so that conduction only occurs for a fraction of the whole RF cycle. If the input signal is increased sufficiently, or a device with a sufficiently low pinchoff is selected, the drain current still swings to I.sub.max in about a half sine wave.
The output current and voltage waveforms are not exact replicas of the input signal waveforms. However, the output signals contain non-fundamental frequency components that can be filtered out. Thus, the classical Class B mode amplifier incorporates a "tank" circuit in parallel with the load resistor. The components of the tank circuit are selected to short-circuit the harmonic components of current. Only the fundamental component of current is allowed to flow through the RF load resistor. The drain efficiency of the amplifier is thereby .pi./4 or 78%.
Further improvement in efficiency is obtained by biasing the gate well beyond pinchoff, in what is termed a Class C mode. However, as the conduction angle is reduced and the efficiency theoretically approaches 100%, the fundamental RF power output decreases in comparison to the Class B or Class A conditions.
Other techniques are also well known for obtaining improved power and/or efficiency. These include non-zero harmonic impedance terminations and/or higher, overdriven input levels. These improvements are summarized by D. M. Snider in an article entitled "A Theoretical Analysis and Experimental Confirmation of the Optimally Loaded and Overdriven RF Power Amplifier", IEEE Trans. Electron Devices, Vol. ED-14, No. 12, Dec. 1967, pp. 851-857. They do not constitute fundamentally different modes of operation, although certain specific cases have been named as Classes D-F.
For instance, Sokal et al., in U.S. Pat. No. 3,919,656, discloses a distinctive high-efficiency mode amplifier that has been termed a Class E amplifier. It consists of a heavily overdriven active device which acts more like a switch than a linear current control. The load network is synthesized such that the RF voltage and current are never non-zero simultaneously. This gives a very high DC to RF conversion efficiency.
These high-efficiency RF amplifiers are characterized by several features. One is that the RF current flow in the active device has a zero value for a significant portion of the RF cycle, causing a reduction in DC power with minimal reduction in RF power. Another is that the RF voltage waveform at the output terminal of a current-source (as opposed to switch mode) active device is sinusoidal, or symmetrical, about the DC supply voltage. Also, the quiescent or average (direct) current drawn by the device is low under conditions of low or zero RF drive. As the RF drive is increased, the DC increases.
The voltage symmetry feature of conventional high efficiency modes becomes a serious limitation for practical implementation when the DC supply voltage is low ("Low" is here interpreted as being of the same order of magnitude as the turn-on, or "knee" voltage of the device). There is therefore a need for an RF amplifier having a high efficiency mode of operation that generates an RF voltage which is several times (rather than just two times) the DC supply, and is therefore well suited to low voltage supply applications.